Most of what this page is about, is how to make sure you use the right tester, in the right way. Only like this, such results are meaningful to discuss about. To make this page not too difficult, anything that supplies details is presented with a link, you are much advised to read those links when you are interested.
What tester to use
Generally, tube testers are divided in parametric testers, and quality testers. The greater part of historical testers are quality testers, whereas such results are difficult to reproduce on two random testers of the same model. Besides these are all 50 years old, and normally uncalibrated by the user. So to allow the user to verify our test results, we use a parametric tester, and supply these test results with our tubes.
What method to test
It doesn't matter wether a new made parametric tester is used, or a vintage model, results should always be the same. However there are ways to produce different outcome, by a different use. Please read following points, to make sure you test the same way as we do, and only then, we guarantee that test results will compare.
Cold anode testing with "impulse test" curve tracers.
This is generally a problem, because such testers fail the capability to heat up the anode at it's full working temperature. This is very tedious to explain, because the manufacturers do not speak of this as a problem, or say it doesn't matter. So when such a tester tested a tube at say 400V, 100mA, the tube was indeed tested with this data, but only for 1/1000 of a second, with an impulse only. This generates no heat in the anode, and the only heat produced is by the cathode heater. So the thermal balance in the tube is another, and mainly the tube as a whole is not fully warm. This means the tube is not at full 100% emission, because when the hot cathode is surrounded by a cold anode, this reduces the cathode temperature considerable. So even when the test results look very impressive, you do need to take this error into acount, in the range of 5...20% error, depending on the tube type and tube condition. Used tubes will have a larger error than new tubes. Yet, being aware of this lower reading, such testers may be used.
Gm (transconductance) must be measured in auto bias.
For reasons of simplicity, most tube testers and software, use fixed bias voltage to test the tubes, however due to natural tolerance, this will result in random plate current. A compare with average plate current (so the data sheet value) will only tell how much deviation there is, and not tell much about the condition of the tube. To avoid this problem, we test the tubes in auto bias, meaning the plate current is always the same. Like this, we achieve the same temperature of the tube always. The test results will be: the grid voltage needed for this, and transconductance resulting. So these two numbers (Ug, and Gm) are the test result, and plate current is always the same. Ug by itself represents the factory tolerance on this, which number is not very meaningful to judge by itself. However a CHANGE of Ug is very important to judge wear out of older tubes. For this, you just set the grid voltage to the same value as when the tube was new, and the loss of plate current will immediately tell the tube condition. It should be obvious, the original, individual Ug voltage of the tube must be used for this, and not the data sheet value, which is only an average number.
However to test transconductance, you need to set the plate current back to the value as written on the box. Reasons are explained in the following text:
Transconductance depends very much on the plate current and also anode heat. For that reason, if a transconductance measurement is done, with the intention to compare it with something, you must always use the same anode heat, and the same plate current. So a transconductance measurement under some "random" conditions can not be compared with the data sheet value, and also not with the individual value of the tubes. In case, you want to compare it with the data sheet value, you need to adjust the tube for the same anode heat, and same anode current as in the data sheet. This is done by adjusting Ua, and Ug until these conditions are achieved. Yet a compare with the data sheet would in the end only show the deviation of the average value, which may be -30% to +40% with factory new tubes, depending on the type and the brand, and in the end this tells not much about in individual tube itself. Much more interesting, is to compare a random EML tube Gm value, with the value it had when it was new. This will directly tell the use condition. For that reason we write the test conditions on the box, which means: Anode Voltage and Anode Current to be set. The setting of Anode Current must be done by changing Ug. After waiting minimum 5 Minutes for thermal balance, Transconductance (Gm) can be tested. This is how we do it at the EML factory, and only like this you can compare tube with out test data.
What is good manufacturer data?
At Emission Labs ®we take pride in presenting absolutely correct tube data, which is written on each tube box in detail.
What is a recommended tube tester?
To say it again, an "impulse tester" like some new made curve tracers, may be used, but these may have a faulty result of 5...20%.
Why we use the AT1000: The only reason why we use the Amplitrex AT1000 is because it can be bought new, and it can just do what we need. There are many AT1000 around, and they work all the same, because the software was never revised ever since 20 years. You do need to be well aware of the limitations this tester has. The default mode is useless for our purpose, and in general too, but we will explain to how to tester can be used in another mode. Test results can be good, if you define clearly what the tester is supposed to do.